Background and Objectives: Aspects of trial design, screening and study efficiency can affect recruitment and the findings of the trial itself. A clear understanding of the screening and study inclusion process will assist clinicians in interpreting trial results. Design: Prospective observational data collection on all patients screened for possible inclusion in a randomized controlled trial of normal vs. augmented renal replacement therapy in critically ill patients (the RENAL Trial). Setting: 35 hospitals in Australia and New Zealand. Participants: All patients screened for the RENAL Trial. Results: We screened 4,551 patients. Of these patients, 767 were ineligible because of lack of inclusion criteria and 2,085 because of exclusion criteria. Of the remaining 1,699, 1,508 (88.7%) were enrolled. The three most common exclusion criteria which prevented recruitment of potentially eligible patients were that the patient had end-stage kidney failure and was already on chronic dialysis (484; 23.2%), the patient's body weight was either <60 or >120 kg (456; 21.8%), and the fact that the patient had already received renal replacement therapy during the index admission. Important modifiable impediments to recruitment were inability to obtain consent in 191 cases, unavailability of research staff in 124 cases, physician objection in 89 cases, and inability to deliver the trial protocol in 78 cases. Conclusion: The RENAL Trial's enrolment efficiency was high and compared favourably with previous large intensive care units trials and with that of trials in patients with acute renal failure. The high rate of enrolment suggests that the results can be applied with confidence to most patients with de novo acute renal failure. The loss of close to 1.5% of patients due to consent issues highlights a common problem in critical care trials. The low rate of physician objection suggests clinical equipoise.